What is the Difference Between Transmembrane and Peripheral Proteins

The main difference between transmembrane and peripheral proteins is that transmembrane protein is a type of integral membrane protein, spanning the entirety of the cell membrane, whereas peripheral proteins are the main type of membrane proteins, which adhere only temporarily to the cell membrane.

Transmembrane and peripheral proteins are two types of membrane proteins present in the cell membrane. Transmembrane proteins form channels and pores to move molecules across the membranes. Meanwhile, the peripheral proteins on the extracellular surface are responsible for cell-cell signalling and interactions. And, the peripheral proteins along the cytosolic surface are responsible for triggering intracellular signalling pathways, anchoring cytoskeletal proteins, etc. 

Key Areas Covered 

1. What are Transmembrane Proteins
     – Definition, Structure, Function
2. What are Peripheral Proteins
     – Definition, Structure, Function
3. What are the Similarities Between Transmembrane and Peripheral Proteins
     – Outline of Common Features
4. What is the Difference Between Transmembrane and Peripheral Proteins
     – Comparison o Key Differences  

Key Terms 

Cell Membrane Proteins, Integral Membrane Proteins, Peripheral Proteins, Transmembrane Proteins

What are Transmembrane Proteins 

Transmembrane proteins are a type of integral membrane proteins. Generally, there are two types of integral membrane proteins. They are transmembrane proteins or integral polytopic proteins and integral monotopic proteins. Basically, transmembrane proteins are the main type of integral membrane proteins, spanning the entire cell membrane. On the other hand, integral monotopic proteins associate with the cell membrane only from one side, but they do not span the cell membrane. Additionally, there are two types of transmembrane proteins as single-pass and multi-pass membrane proteins based on the number of times they cross the membrane.

Figure 1: Transmembrane Proteins

Furthermore, the hydrophobic side chains of transmembrane proteins are responsible for interacting the fatty acyl groups in the lipid bilayer, which are also hydrophobic. Also, these membrane-spanning domains can be either α helices or multiple β strands. Additionally, the domains extending to the aqueous medium of either side of the cell membrane are hydrophilic. The main function of the transmembrane proteins is to serve as the gateways to permit the transport of selected substances across the cell membrane. These gateways or channels can be opened or closed by undergoing conformational changes in the transmembrane protein.  

What are Peripheral Proteins 

Peripheral proteins are one of the two types of membrane proteins. Generally, there are two types of membrane proteins. They are integral membrane proteins and peripheral proteins. Here, integral membrane proteins are permanently bound proteins to the cell membrane either completely or incompletely spanning the cell membrane. On the other hand, peripheral proteins temporarily attach to the lipid bilayer or to the integral proteins by combinations of hydrophobic, electrostatic or other non-covalent interactions. 

Figure 2: Membrane Proteins

Moreover, peripheral proteins localize on both surfaces of the cell membrane; either on the extracellular surface or the cytosolic surface. These exoplasmic peripheral proteins play a key role in cell signalling and cell-cell interactions. On the contrary, the peripheral proteins on the cytosolic surface are mainly the enzyme protein kinase C, which takes part in the signal transduction. In contrast, some of the cytosolic peripheral proteins are cytoskeletal proteins.  

Similarities Between Transmembrane and Peripheral Proteins 

• Transmembrane and peripheral proteins are two types of proteins that occur in the cell membrane. 
• They are a common type of proteins in the body. 
• Generally, 20-30% of genes in the genome are encoded for membrane proteins. 
• Moreover, they perform a number of functions vital to the survival of an organism. 
• Both types of proteins are also important as drug targets in medicine. 

Difference Between Transmembrane and Peripheral Proteins 

Definition 

Transmembrane proteins refer to a type of integral membrane proteins, which cross the membrane and can act as pathways for ions and molecules, while peripheral proteins refer to a type of membrane proteins, which adhere only temporarily to the biological membrane with which they are associated. Thus, this is the main difference between transmembrane and peripheral proteins.

Correspondence 

Furthermore, transmembrane proteins are one of the two types of integral membrane proteins, while peripheral proteins are one of the two types of membrane proteins. 

Intrinsic or Extrinsic 

Another major difference between transmembrane and peripheral proteins is that transmembrane proteins are intrinsic proteins, while peripheral proteins are extrinsic proteins. 

Occurrence in the Cell Membrane 

Moreover, the transmembrane proteins span the entirety of the cell membrane, while the peripheral proteins occur in both the extracellular surface and the cytosolic surface of the cell membrane. 

Type of Binding to the Cell Membrane Components 

Additionally, transmembrane proteins permanently bind to the cell membrane components while peripheral proteins temporarily bind to the components of the cell membrane. 

Method of Binding 

Also, the hydrophobic side chains of transmembrane proteins interact with fatty acyl groups of membrane phospholipids while peripheral proteins form indirect interactions with the head groups of membrane lipids or direct interactions with the integral membrane proteins.  

Method of Dissociation 

Besides, transmembrane proteins can be dissociated from the cell membrane by using detergents, nonpolar solvents, or sometimes denaturing agents while peripheral proteins can be dissociated from the cell membrane by treating with a polar reagent, such as a solution with an elevated pH or high salt concentrations.  

Function 

We can also identify a difference between transmembrane and peripheral proteins in terms of their functions. Transmembrane proteins form channels and pores to move molecules across the membrane. Meanwhile, the peripheral proteins on the extracellular surface are responsible for cell-cell signaling and interactions. But, the peripheral proteins along the cytosolic surface are responsible for triggering intracellular signaling pathways, anchoring cytoskeletal proteins, etc. 

Conclusion 

Basically, transmembrane proteins are a type of integral membrane proteins, completely spanning the cell membrane. Also, they serve as channels and pores, which facilitate the movement of hydrophilic molecules in and out of the cell membrane. On the other hand, peripheral proteins are a type of membrane proteins, which bind either to integral membrane proteins or phospholipids. Moreover, they occur in both surfaces of the cell membrane, including the extracellular and cytosolic surfaces. Additionally, they perform many functions in the cell, such as involving in signaling pathways. However, the main difference between transmembrane and peripheral proteins is their occurrence and function in the cell membrane. 

References:

1. Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Section 3.4, Membrane Proteins. Available Here.

Image Courtesy:

1. “Transmembrane proteins” By Meng-jou wu at English Wikibooks – Transferred from en.wikibooks to Commons by Adrignola using CommonsHelper. (Public Domain) via Commons Wikimedia   
2.”Membrane protein” By Meng-jou wu at English Wikibooks – Transferred from en.wikibooks to Commons by Adrignola using CommonsHelper. (Public Domain) via Commons Wikimedia